Laser-produced plasma (LPP) devices are an attractive source of X-rays or short-wavelength radiation due to their relative small size, high brightness and high spatial stability. Two established applications for LPP are microscopy and lithography. Within such LPP devices, one or more zone plates are often employed as an x-ray condenser in the optic area designed to focus the generated radiation. In addition, zone plates may also be used as an x-ray condenser optic in a microscope or similar instrument.
Zone plates, such as Fresnel zone plates, may be high transmission, low aberration, and are reasonably simple to fabricate when compared to available optics in the x-ray portion of the spectrum. A conventional zone plate design is comprised of an absorbing geometry patterned on the top surface of a highly transmissive membrane. Since such a zone plate is optimized by maintaining the highest contrast between the absorbing geometry and the adjacent non-absorbing regions, the supporting membrane is necessarily very thin and likewise fragile.
Unfortunately, under intense or prolonged illumination conditions (i.e., radiation exposure), the zone plate optic can suffer from radiation damage. As the dose increases, mechanical integrity can be compromised leading to increased aberrations and eventual failure of the zone plate. Specifically, the integrity of the highly transmissive membrane supporting the structure typically begins to fail after prolonged radiation bombardment.
Designs and fabrication methods that limit susceptibility to radiation damage can extend the useful working life; however, conventional approaches to date are limited in this regard. For example, conventional design soften use a flat, solid membrane supporting the entire zone plate. The radiation passing through the zone plate bombards portions of the supporting membrane not protected by a radiation absorbing material. Over time, these areas of the membrane can begin to fail because of exposure, which may result in the entire zone plate failing within the LPP device.